1. Field of the Invention
The present invention relates to the field of analysis of a test sample. More particularly, it relates to the analysis of a test sample for the presence of a sugar, such as glucose, fructose, lactose, galactose, mannose, maltose or the pentoses.
The analysis of test samples for the presence of sugars finds utility in many unrelated arts. Thus, the present invention pertains to such diverse technologies as the brewing industry, biochemical research and medical diagnostics. In the brewing industry, for example, starch is converted to sugars, such as maltose, prior to actual fermentation. The presence of maltose is therefore carefully monitored to assure high yields from the grain starting material. Many biochemical systems require glucose in carefully controlled concentrations as their cellular energy source, and the research of such systems necessitates that these concentrations be carefully monitored. The medical profession utilizes sugar analysis to a great extent in diagnosing and controlling such diseases as diabetes mellitus, which manifests itself by abnormally high glucose concentrations in the blood and urine.
Thus, the field of the present invention extends to a very diverse assortment of pursuits. It finds applicability wherever sugar analysis becomes a matter of significance, be it in brewing, the food industry, scientific research or medicine.
2. Description of the Prior Art
The history of sugar analysis is perhaps most noteworthy because it has seen dramatic change over the years, both in the basic chemistries utilized and in its format. For the most part these analyses can be characterized as oxidizing systems which, when reduced, initiate reaction conditions leading to a detectable response, such as a color change or change in wavelength of ultraviolet light absorbed or reflected by the system. Thus, reducing sugars will convert silver oxide to metallic silver, and, if a solution of the sugar is applied to a piece of filter paper impregnated with silver oxide, a black dot develops. F. Feigl, Chem. Ind., Vol. 57, p. 1161, London (1938). Similarly, o-dinitrobenzene and the 3,4- and 3,5-isomers of dinitrophthalic acid give a sensitive color reaction (forming violet shades) when heated with reducing sugars in Na.sub.2 CO.sub.3. T. Monmose, et al., Chem. Pharm. Bull. Tokyo, Vol. 12, p. 14 (1964); F. Feigl, Spot Tests in Organic Analysis, 7th Edition, pp. 338-339, Elsevier Publ. Co., New York (1966).
But as early as 1849 it was known that reducing sugars would cause an alkaline solution of CuSO.sub.4 to precipitate the yellow to red Copper (I) oxide (or oxyhydrate). H. Fehling, Ann., Vol. 72 (1849). See also B. Herstein, J. Am. Chem. Soc., Vol. 32, p 779 (1910). This early milestone, known as the Fehling test, lent impetus to the development of a far more sensitive test which utilized silver oxide in ammonia, the so-called Tollens reagent, which reacts readily with reducing agents to produce a black precipitate of metallic silver, often forming a mirror on the inside walls of glass reaction vessels. B. Tollens, Ber., Vol. 14, p. 1950 (1881); Vol. 15, p. 1635, 1828 (1882).
Because of the relatively high incidence of diabetes mellitus and its accompanying serious clinical consequences, high interest from the biological and medical professions arose in new techniques for analyzing glucose levels in urine and serum. This keen interest led to the development of several new procedures which deviate dramatically from their solution chemistry forbears. These utilize sophisticated biochemical systems which can be incorporated into dry, dip-and-read devices, used in solution or suspension techniques, or in conjunction with spectrophotometers and other hardware.
Of these new techniques, the present invention lends itself to an enzymatic system wherein the analyte (for instance glucose) is a substrate for a particular enzyme, the reaction products being capable of eliciting a detectable response from a family of indicator compounds known loosely in the art as "benzidine-type indicators". These will be more carefully defined, infra, but for the present suffice it to say these compounds undergo color changes in the presence of hydrogen peroxide and the enzyme peroxidase. The glucose/glucose oxidase system exemplifies the prior art, wherein glucose is oxidized to gluconic acid with the concomitant formation of H.sub.2 O.sub.2 in accordance with ##STR2##
It is the concomitant formation of hydrogen peroxide which facilitates the subsequent, indicator-related steps leading to observable color formation or other detectable response. Thus a benzidine-type indicator responds in the presence of hydrogen peroxide and peroxidase by changing its light absorptive capability.
In practice, this technology is presently utilized for glucose analysis in the form of dip-and-read reagent strips such as those marketed by the Ames Division of Miles Laboratories, Inc. under the trademark CLINISTIX.RTM. and others. Broadly, these comprise a plastic strip, at one end of which is mounted an absorbent paper portion impregnated with the appropriate enzymes, indicator compound and buffering agents as the principal active ingredients. They are used by dipping the reagent-bearing end into the test sample, removing it and comparing any color formed in the paper with a standard color chart calibrated to various glucose concentrations.
Although the mechanism of color formation from benzidine-type indicators in the presence of H.sub.2 O.sub.2 and peroxidase is not known to a certainty, it is known that two sequentially occurring colorforms result: a first species which is blue in color, and a second which is brown. Because the blue species tends to be transient, ultimately metamorphosing to the brown, it is necessary to look for the color change within a prescribed time period. Otherwise the true significance of color change is lost, as subtle shades of blue--which are easily distinguishable--give way to the less easily interpreted brown hues. The higher the sugar concentration in the test sample, the more aggravated this problem becomes due to the limiting effect on capacity to detect the higher ranges of sugar concentrations. Thus, it can be seen that it is highly advantageous to extend the duration of the blue species, thereby permitting greater differentiation between sugar concentrations, as well as providing higher limits to the detectable concentration ranges.
Moreover, because analytical tools such as reagent strips are not used immediately after manufacture, but are usually stored for relatively long periods, and because too long a period between manufacture and use can result in a loss in efficacy, enhanced shelf life can be a marked asset: the better the shelf life, the more dependable the analytical results.
Thus, the present invention relates to a sugar-sensitive composition utilizing the above-described enzyme/benzidine-type indicator system of the prior art. But the invention goes beyond this system by providing improved stability of the intermediate blue colorform, as well as enhanced storage capability, i.e., shelf life.
Several patents have issued which are deemed pertinent to the present invention. U.S. Pat. No. 2,848,308, issued to Alfred H. Free discloses the basic enzyme chemistry whereby glucose oxidase, peroxidase and a benzidine-type indicator are used in a reagent strip to determine glucose in urine or other bodily fluid. U.S. Pat. No. 3,753,863, issued to Speck discloses the use of lower alkane polyols to "stabilize" indicator solutions of the benzidine type. Finally, U.S. Pat. No. 4,071,317, issued to Lam, discloses the stabilization of an occult blood-sensitive composition through the use of certain sulfone, sulfoxide and amide compounds as diluents during preparation of the composition. This latter composition comprises an organic hydroperoxide compound, and an indicator compound such as of the benzidine type.
To summarize the state of the art prior to the present invention, sugar-sensitive chemistries began to appear on the analytical scene as early as the middle of the 19th century while the advent of Fehling's solution and Tollens' reagent. Most of the "purely chemical" systems which have since emerged have been largely superseded by biochemical systems, particularly those which comprise a sugar oxidase, peroxidase and a peroxide-sensitive indicator of the benzidine type. These latter indicator compounds have been said to be stabilized by the presence of lower alkyl polyols. Finally, a composition sensitive to the presence of occult blood in urine is taught to be stabilized if formulated in the presence of certain sulfone, sulfoxide and/or amide compounds. There is no teaching, to applicants' knowledge, anywhere in the prior art suggesting the presently disclosed and claimed sugar-sensitive composition and test device, or method for their use.